Citric acid has been commercially removed from aqueous solutions thereof, such as fermentation beer, by the procedure of adding lime to form a precipitate of calcium citrate and then reacting the calcium citrate with aqueous sulfuric acid to form citric acid solution and insoluble calcium sulfate. This procedure has the disadvantages of requiring expendable lime and sulfuric acid as well as disposal of calcium sulfate.
Liquid-liquid extraction procedures have also been proposed in the prior art. Amines, mixtures of amines and hydrocarbons, and amine salts have been proposed. One commercial procedure employs a mixture of amines and hydrocarbons, for example, and is disclosed in British Pat. No. 1,426,018. The recovery of citrate salts using an amine-organic solvent mixture for extraction of citric acid is also disclosed in U.S. Pat. No. 3,944,606.
U.S. Pat. No. 3,304,157 discloses the use of various amides, including alkyl amides, for extraction of phosphoric acid. It is known, however, that some materials, such as alcohols, which are suitable for the extraction of phosphoric acid are not suitable for extraction of citric acid.
None of the known prior art discloses or suggests the use of N-substituted alkyl amides for the extraction of citric acid.